This invention relates generally to amplitude modulators and more particularly to an electro-optic modulator for generating solitons from a continuous wave signal.
Long distance optical transmission using optical amplifiers can provide greater bandwidth at lower cost than that using electronic regeneration. Erbium doped optical fiber amplifiers can easily handle several channels simultaneously, and do so with low crosstalk. For long distance transmission, it is necessary to use a transmission mode which is resistant to the various dispersive effects of the fiber. In au optical fiber transmission path, the optical fiber""s chromatic dispersion, acting by itself, attempts to broaden pulse signals in time. The fiber""s index, which also depends on the intensity of light, acting by itself through the process of self phase modulation, always serves to broaden the pulse""s frequency spectrum. Thus, for long distance transmission, an optical signal which is resistant to the various dispersive effects of the optical fiber can result in an increase in the spacing between optical amplifiers in the optical transmission path.
Under certain conditions such as, for example, zero loss or loss periodically compensated by optical gain, a soliton is nondispersive in the time domain. Thus, the waveshape of a soliton is independent of the distance that it travels along an optical fiber. In addition, a soliton is also nondispersive in the frequency domain. Thus, for a range of soliton pulse widths, such as 50-80 ps for a data rate of 2.5 G b/s, and fiber group delay dispersion parameters of approximately 0.7-2 ps/nm/km, the distance that a soliton can be transmitted before serious dispersive effects occur is typically 500 km or greater.
Creation of soliton pulses is dependent upon proper launch and transmission characteristics such as pulse power, pulse width, center frequency, and fiber dispersion. Of particular concern for the present purposes, creation of solitons require the generation of temporally narrow pulses, typically on the order of 1-10 picoseconds. These characteristics of solitons are well known to those skilled in the art and will not be discussed further herein. For additional information concerning soliton generation and soliton transmission, see Optical Fiber Telecommunications II, ed. S. E. Miller et al., p.90 et seq. (Academic Press 1988).
One device for generating solitons consists of a high speed amplitude modulator such as an electro-optic waveguide modulator. One class of electro-optic modulators are made of ferroelectric materials, such as z-cut lithium niobate (LiNbO3) or lithium tantalate (LiTaO3). These modulators convert an applied voltage to an optical signal. Typically, an electric pulse is used to generate an optical pulse. Lithium niobate modulators are commonly employed because they offer high speed, a high extinction ratio, and a controllable (or zero) chirp. However, one problem with such modulators is that it is difficult to generate extremely narrow electrical pulses that can be translated into optical pulses of sufficiently narrow temporal width to form solitons.
Therefore, it is desirable to provide an electro-optic amplitude modulator with an electric signal that allows the modulator to generate temporally narrow optical pulses.
In accordance with the present invention, a method and apparatus is provided for generating optical pulses with an electro-optic amplitude modulator. The modulator includes first and second waveguides that form an optical interferometer. At least the first waveguide includes an electro-optic material such as lithium niobate and an electrode extending along a portion thereof. Input and output optical waveguides are respectively coupled to input and output junctions of the interferometer. A voltage source biases the electrode such that a modulation switching curve arises that generates two optical pulses over a complete voltage cycle.
In accordance with another aspect of the invention, a method is provided for generating optical pulses with an electro-optic amplitude modulator having a pair of waveguides and at least one pair of electrodes for controlling a refractive index of at least one of the waveguides. In particular, a cw optical signal is received at an input waveguide of the modulator. At least one electrical pulse is applied to the electrode pair to modulate the cw optical signal so that an edge of the electrical pulse yields an optical pulse at an output waveguide of the modulator. The optical pulse may have a temporal width substantially equal to the temporal width of the edge of the electrical pulse.
In contrast to known biasing arrangements in which an electrical pulse was required to produce an optical pulse, the present invention advantageously produces an optical pulse upon a change in voltage levels. Since it is relatively easy to produce sharp voltage transitions (as opposed to narrow electrical pulses), the invention is capable of produces extremely narrow optical pulses, such as solitons, for example.